It is known that when the main rotor turns, it sucks in air from upstream and blows it out downstream like a propeller, thereby allowing the helicopter to fly up and down. When flying forwards, the rotor acts both as a propeller and as a wing. As a propeller it accelerates the mass of air that passes through it so as to create a moving slipstream, with the pressure and the speed of the air varying along said slipstream. Acting as a wing, the rotor causes the slipstream to be deflected.
The stream of air downstream from the main rotor, commonly referred to as the “slipstream”, is disturbed in part by the main elements for rotating the main rotor and by certain fairings, or indeed the helicopter fuselage itself, in particular when carrying external loads or during special flight configuration.
The main rotor is set into rotation by a driving force applied to its shaft. For this purpose, the engine on board the helicopter drives the shaft via an appropriate mechanical assembly. This leads to equal and opposite torque being applied to the fuselage, and this torque needs to be compensated by means of an auxiliary device such as a tail rotor, which is generally also driven by the same engine.
In addition the fuselage and the rotors, it is also known that a helicopter also includes one or more substantially horizontal stabilizers and one more substantially vertical tail fins. These elements are for the most part located at the rear of the fuselage and serve to provide the helicopter with control, stability, and the ability to maneuver about two perpendicular axes. It should be observed that the horizontal stabilizers and the tail fin may sometimes be constituted in the form of a single assembly of T-shape or of cross-shape (+). Similarly, the vertical tail fin may be formed by a single aerodynamic surface or it may be in the form of two aerodynamic surfaces forming a V-shape, for example. Another solution consists in placing a substantially vertical tail fin at the outside end of a substantially horizontal stabilizer. Nevertheless, these examples are not limiting.
The tail fin and the stabilizer are generally stationary and are consequently located at the rear end of the fuselage (in a zone referred to as the “tail boom” by the person skilled in the art), and they are to be found in a zone which is subjected at least in part to the air flow or slipstream coming from the main rotor and from the fuselage.
In practice, the main rotor acts like an aerodynamic exciter. Thus, its slipstream is turbulent. Turbulence corresponds to variations in pressure, speed, and angle of incidence of the aerodynamic flow that are distributed over quite a broad range of relatively high frequencies.
The slipstream behind the main rotor of a helicopter is pulsed at a fundamental frequency equal to the product b×Ω where b is the number of blades of the main rotor and Ω is the speed of rotation of said rotor.
Nevertheless, frequencies which are harmonics of b×Ω can sometimes also appear.
Under such conditions, the tail fin and the stabilizer are subjected simultaneously to said aerodynamic excitation which leads directly to exciting resonant modes of the helicopter structure. This phenomenon is generally known as “tail shake”.
Furthermore, during certain stages of flight (e.g. during quartering flight), it is possible that the “tail shake” phenomenon is caused not by the slipstream from the main rotor but by the slipstream from the fuselage. A helicopter fuselage often carries external items (winches, missiles, torpedoes, auxiliary tanks, . . . ) which have the effect of spoiling (increasing drag and turbulence) the air flow from the fuselage itself. The turbulence which is the main cause of said tail shake can be small or negligible during nominal flight (cruising flight in a calm atmosphere), but can become much stronger during certain stages of flight (quartering flight, flight in a turbulent atmosphere, . . . ).
Even if the aerodynamic excitation is relatively small, it can lead to a level of vibration that is disagreeable in the cockpit and in the passenger cabin and that is harmful for the structure as a whole and for the mechanical elements of the helicopter.
The vibration as generated in this way can be distributed over the various axes of the structure as a function of where the slipstream strikes. For example, if it is applied to the vertical tail fin, that will generate an effect that is mainly lateral, and in particular it will excite a first mode of resonance in lateral bending of the tail boom. Conversely, if the slipstream reaches the horizontal stabilizer, then the vibration will be mainly vertical, thereby exciting the first resonant mode of the tail boom in vertical bending.
The various kinds of vibration due to the first lateral and/or vertical bending mode of the structure of the helicopter, and possibly also to a resonant mode in twisting of the helicopter, all present numerous drawbacks, including the following:                a reduction in crew and passenger comfort;        material and equipment fatigue; and        problems associated with operating on-board equipment such as weapons systems when the helicopter is fitted therewith.        
In an attempt to provide a solution to this problem, documents FR 2 737 181 and U.S. Pat. No. 5,816,533 disclose a method and a device for generating an effect that opposes specifically vertical vibration, by applying alternating variation to the pitch of the blades in the tail rotor of the helicopter, which rotor, for this purpose, needs to be inclined relative to the plane of symmetry of the helicopter containing the longitudinal axis and the vertical axis of the helicopter.
Nevertheless, it has been found that the solution according to those patents FR 2 737 181 and U.S. Pat. No. 5,816,533 presents a first drawback associated with the fact that the aerodynamic excitation leads to excitation of resonant modes of the fuselage structure, and the blades of the tail rotor are controlled in order to reduce the vibration that results therefrom at certain particular points of the helicopter. Unfortunately, that action tends to shift the vibration nodes and anti-nodes along the structure, but without thereby eliminating excitation of the resonant modes of said structure.
As a result, the tail fin and the horizontal stabilizer, in particular, are subjected to the bending of the structure where they are attached thereto, depending on the way the structure responds to the excitation.
A second drawback of the system described in those patents lies in an increase in the level of noise that is generated because of the variations in the characteristic parameters (speed, pressure, . . . ) of the air flow through the tail rotor as generated by varying the angle of incidence of its blades. These effects are harmful for the environment and raise severe problems in terms of regulations.
It should also be observed, by way of example, that another drawback due to changing the angle of incidence of the blades of the tail rotor for opposing the above-mentioned vibration lies in said variations in the angle of incidence of the blades of said rotor generally generating alternating forces and moments which reduce the lifetime of the assembly of parts constituting the rotor and the means for driving it in rotation.